A recent development in integrated circuit technology has been the introduction of circuits incorporating both field effect transistors and polysilicon (polycrystalline silicon) emitter bipolar transistors on a common semiconductor substrate chip. Techniques of this type, which is becoming known generally as `merged technology` are described for example in our UK specification No. 2173638 (P D Scovell et al. 15-13-8X). Whilst this technique represents a considerable technical advance by combining on a single chip the advantageous features of field effect and bipolar devices, its full potential in the custom circuit field is restricted by the requirement, prior to the provision of any custom features, to define those device regions that are to become field effect transistors and those that are to become bipolar transistors. The definition of device types at an early stage of processing places constraints on the nature of the circuit which can be provided by a subsequent customising process to interconnect the devices in a particular way.
The object of the invention is to minimise or to overcome this disadvantage.